Studies were made concerning the natural mechanisms for transmission perpetuation and evolution of type C viruses and virogenes in mammals, especially primates. The mechanism of expression and regulation of these genes has been explored with the identification and preliminary characterization of several peptides and factors which effect phenotypic changes typical of malignancy. Several new primate and murine endogenous viruses were isolated from Asian mice and primates closely related to humans. These primate viruses will allow the development of specific reagents with which human cells may be examined for the expression of viral related genetic material.